1. Field of the Invention
The present invention relates to a method for three-dimensionally correcting distortions of an image dataset and a magnetic resonance apparatus for implementing the method.
2. Description of the Prior Art
Magnetic resonance is a known technique for acquiring images of the inside of a body of an examination subject. In a magnetic resonance apparatus, rapidly switched gradient fields that are generated by a gradient system are superimposed onto a static basic magnetic field, which is generated by a basic field magnet system. Furthermore, the magnetic resonance apparatus has a high-frequency system, which irradiates high-frequency signals into the examination subject for triggering magnetic resonance signals and which receives the generated magnetic resonance signals, on the basis of which image datasets are prepared.
In magnetic resonance imaging, sufficient homogeneity of the basic magnetic field constitutes a determining factor for the quality of the magnetic resonance images. Non-homogeneity of the basic magnetic field, within an imaging volume of the apparatus, thereby causes geometric distortions of the magnetic resonance image, which is proportional to the field non-homogeneity. The same is true for non-linearities of the gradient fields.
A distortion-free and location-accurate imaging is important for many applications, for example when magnetic resonance images are used for planning radiotherapy of tumors or for preparing or performing a surgical intervention.
Magnetic resonance apparatuses are known which have an examination space for accepting the examination subject, such as a patient, which can be accessed from all sides for the purpose of intraoperative interventions and which is large and openly designed for accommodating claustrophobic patients. In apparatuses of this type, the problem of distortions is intensified as a result of the construction, particularly at the edges of the imaging volume and in apparatuses having an intense basic magnetic field.
A shim device is a known measure for improving the basic magnetic field homogeneity. Shim coils are used for this purpose, which homogenize the basic magnetic field when suitably operated with direct currents. As is known from German Patent 195 11 791, the basic magnetic field can be described within the imaging volume with coefficients of a spherical function series expansion. The shim coils are normally fashioned such that they essentially compensate a specific inhomogeneous field portion corresponding to one of the coefficients. A corresponding correcting measure is not utilized for the gradient fields.
For example, U.S. Pat. No. 5,099,208 discloses a pick-up method for magnetic resonance images wherein, despite non-homogeneity of the basic magnetic field, an image dataset having only little distortion is produced by a combination of two image datasets, which are generated by specific pulse sequences. For this purpose, a first image dataset is produced from an area of the examination subject to be imaged by a first pulse sequence. Subsequently, a second image dataset is produced from the same area to be imaged by a second pulse sequence, which differs from the first pulse sequence only by virtue of exchanged operational signs. Finally, a third image dataset free of distortions is obtained by combining two image datasets. In the aforementioned method, the obtainable exactness of the distortion correction, however, is limited to 2 to 3 mm topical resolution. Furthermore, such a course of action cannot be performed for all pulse sequences, and the pick-up time is doubled, since two image datasets are picked up for acquiring a magnetic resonance image with little distortion, so that only a reduced patient throughput can be achieved.
A method for correcting distortions as a result of non-homogeneity of the basic magnetic field and/or non-linearities of the gradient is disclosed in German Patent 198 29 850 which utilizes coefficients of the spherical function-series expansion for the basic magnetic field and/or for the gradient fields in order to eliminate distortions of an image dataset. At least coefficients describing field non-homogeneity of the basic magnetic field and/or nonlinear field components of the gradient fields are used. The precision of the correcting method depends on the exact knowledge of the aforementioned coefficients. The coefficients are determined using as phantom. For this purpose, the phantom is positioned in the imaging volume of the magnetic resonance apparatus and magnetic resonance images of the phantom are picked up. The coefficients are determined from a partially automatic evaluation and partially manual evaluation of these magnetic resonance images. Since the entire imaging volume of the magnetic resonance apparatus is acquired by the phantom, it is comparatively large with approximately 0.25 m3, comparatively heavy with more than 20 kg and therefore is correspondingly difficult to handle. As a result of component drift in the magnetic resonance apparatus, the pickups with the phantom are repeated every two to three weeks for determining the coefficients. The apparatus is unavailable for examining patients during the repetitions of the pickups. Moreover, only a precision of 2 to 3 mm spatial resolution can be obtained for correction of distortions using coefficients that have been determined on the basis of the phantom.
An object of the present invention is to create an improved method for precisely three-dimensionally correcting distortions of an image dataset and a magnetic resonance apparatus for implementing the method, which, among other things, alleviates the aforementioned disadvantages of known methods and apparatuses.
The above object is achieved in accordance with the principles of the present invention in a method for three-dimensionally correcting distortions of an image dataset obtained in a magnetic resonance apparatus having a basic field magnet system and a gradient system, wherein coefficients of a spherical function-series expansion of the basic magnetic field and/or of the gradient field are utilized for correcting the distortions, and wherein these coefficients are determined in a design and/or production procedure for the basic magnetic field system and/or the gradient system.
The above object also is achieved in a magnetic resonance imaging system designed to implement the above-described method.
Since coefficients are used, which, due to the inventive method, are very precisely determined in the framework of a design (development) and/or production procedure for the basic field magnet field and/or gradient system, they are also available with high precision for the method for three-dimensionally correcting distortions. Among other things, high precision can be obtained during the correction of distortions as a result thereof. Furthermore, magnetic resonance pickups of a phantom for determining coefficients therefore are not necessary. The disadvantages that are associated with the aforementioned phantom method therefore do not occur. It is also not necessary to repeat pulse sequences, which extends the pickup time, for eliminating distortions.